Human B cells express a multiplicity of cell surface antigens that are useful markers for identifying different subpopulations of bone marrow-derived (B) cells and their malignant counterpart in leukemias and lymphomas. We have prepared an anti-human B-cell antiserum (BDA) that reacts with one or more differentiation antigens on B-cells from all normal lymphoid compartments. The antigen(s) is distinct from other known cell surface constituents on human B cells (i.e., Ig, Fc, C, Dr). Of clinical interest is the observation that this antiserum reacted with the majority of neoplastic lymphocytes from some leukemic patients (both acute and chronic type), even though these lymphocytes expressed little or no surface immunoglobulin (a usual characteristic of B lymphocytes) and did not form spontaneous E-rosettes (a marker for identifying T cells). This reagent thus has potential clinical application for identifying a determinant that serves as a marker for certain lympho-proliferative malignancies. In this study, we propose to use immunochemical techniques to first isolate and further characterize the nature of the B cell antigen(s) on normal and leukemic B cells that react with this specific anti-B-cell antiserum (BDA) and second, isolate and identify other B-cell differentiation antigens. The methodology to identify and characterize lymphocyte differentiation antigens is developed and we have employed these methods to identify two differentiation antigens on normal and cultured human thymus-dependent (T) lymphocytes. We plan to use the same strategy for further characterizing the B-cell antigen(s) reacting with this specific B-cell antiserum as well as other conventional and monoclonal B-cell antisera.